The description is based on a method for reducing conductor track spacings of a printed circuit board and on a printed circuit board with a circuit arrangement with reduced conductor track spacings according to the generic type of the main claim.
Switched-mode power supplies may be subdivided into two groups in principle, DC-insulated and non-insulated switched-mode power supplies. Insulated switched-mode power supplies have an input part and an output part which is electrically insulated from said input part. The input part carries mains voltage and is configured such that it cannot be touched by a user.
For handling and cost reasons, the output is usually designed to be insulated from the mains voltage. The insulated output voltage may lie in the SELV range (up to 120 V), but also have higher voltages.
The insulated output voltage allows a considerably reduced expenditure on insulation in LED modules which is highly advantageous for thermal reasons and allows compact designs.
Especially in the case of switch-mode power supplies which are provided for LED illumination, insulated SELV-compliant switched-mode power supplies are very common since the LEDs require adequate cooling at relatively high powers and are sometimes mounted on relatively large metallically conductive heat sinks which may be touched from the outside. In order to be able to comply with the relevant safety standards, such as IEC 61347-1, safety spacings are provided between the input part, which cannot be touched, and the output part (which may be touched), it being necessary to maintain said safety spacings in the form of a creepage distance.
FIG. 1 shows a schematic illustration of a design of an insulated switched-mode power supply according to the prior art which complies with the safety standard according to IEC 61347-1. The input part PRI, which is fed by the mains voltage US, is separated from the output part SEC by a safety spacing S. The creepage distances which have to be maintained in accordance with IEC 61347-1 are defined depending on the maximum occurring voltage between the corresponding components.
FIG. 2 shows, by way of example, a detail of a printed circuit board layout of a design according to the prior art which complies with the safety standard according to IEC 61347-1. The right-hand part of the layout includes the input part PRI, which has mains voltage-carrying conductive parts (components, conductor tracks, . . . ), and is mechanically configured such that voltage-carrying parts cannot be touched from the outside. This may be achieved by corresponding design of the housing and the connection terminals of the switched-mode power supply. The output part SEC is arranged on the left-hand side of the layout. The output part is configured such that it may be touched since a safe extra-low voltage is produced at its output, and the LED modules which are to be operated by the switched-mode power supply, for example, may be touched.
A creepage distance according to IEC 61347-1 is provided between the input part PRI and the output part SEC. This standard prescribes particular creepage distances on the basis of the voltages between two components between the input part and the output part.
FIG. 2 illustrates, by way of example, different required creepage distance spacings with reference to the voltage which is applied between the corresponding conductive parts.
The maximum occurring first voltage U1 between the corresponding components is, for example, 432 V. According to IEC 61347-1, a minimum creepage distance of 8.64 mm between the corresponding components is provided for this voltage.
The maximum occurring second voltage U2 between the corresponding components is, for example, 378 V. According to IEC 61347-1, a minimum creepage distance of 7.56 mm between the corresponding components is provided for this voltage.
The maximum occurring third voltage U3 between the corresponding components is, for example, 544 V. According to IEC 61347-1, a minimum creepage distance of 10.8 mm between the corresponding components is provided for this voltage.
The maximum occurring fourth voltage U4 between the corresponding components is, for example, 170 V. According to IEC 61347-1, a minimum creepage distance of 3.4 mm between the corresponding components is provided for this voltage.
Especially in the case of flyback converters, high voltages may occur in some components for functional reasons and, in practice, this may lead to large required creepage distances at the third voltage U3. This increases the required installation volume of the switched-mode power supply and is therefore undesirable. Specifically in the case of switched-mode power supplies in illumination engineering, it is usual to use so-called add-on boards in the case of relatively complex designs, said add-on boards being fitted on the main circuit board vertically in order to save length on the main circuit board. Here, relatively small creepage distances could make space for components, and add-on boards could be dispensed with.